The present invention relates to hydrostatic transmission (“HST”) systems and controls therefore, and more particularly, to closed loop hydrostatic transmission systems including a charge pump which serves as the source of make-up fluid to the closed loop, and as the source of control fluid to the system controller.
A typical closed loop hydrostatic transmission system of the type to which the present invention relates is illustrated and described in U.S. Pat. No. 4,936,095, assigned to the assignee of the present invention and incorporated herein by reference. In such a closed loop HST system, the closed loop and the various system components are protected from excessive pressures by means of a pair of high pressure (“overpressure”) relief valves, which are typically included in a valve assembly disposed in the pump endcover. As is well known in the art, one high pressure relief valve (HPRV) protects a first side of the closed loop (dumping to the second side when the first side exceeds the HPRV setting), and the other HPRV protects the second side of the closed loop (dumping to the first side when the second side exceeds the HPRV setting).
In the closed loop HST system sold commercially by the assignee of the present invention, there is provided a pair of valve assemblies, each of which includes one of the high pressure relief valves (HPRV's) mentioned previously. By way of background and explanation only, each of these valve assemblies would typically be disposed in a valve bore, and each bore would be in communication with three different locations of the closed loop HST system as follows: (1) the charge pump; (2) one of the servos by means of which the pump displacement is varied; and (3) one side of the closed loop (i.e., one of the conduits extending between the pump and the motor).
Typically, each of the valve assemblies includes, in addition to the high pressure relief valve, a check valve which prevents backflow from the adjacent portion of the hydrostatic closed loop (when it contains high pressure) to the charge pump, while permitting flow from the charge pump to the adjacent portion of the hydrostatic loop (when it contains low pressure). Each of the valve assemblies also would normally include a pressure override (“POR”) valve, by means of which pressurized fluid may be communicated from the adjacent portion of the hydrostatic loop (when it contains high pressure) to the associated servo, thereby reducing pump displacement, whenever the pressure in the hydrostatic loop has exceeded the POR setting.
In addition, the typical closed loop HST system includes a bypass valve assembly which is capable of “crossporting” (or short circuiting) the hydrostatic closed loop. One of the primary uses of the bypass valve assembly arises whenever the vehicle is disabled and must be moved (such as by being towed) some distance. The bypass valve assembly usually functions by means of some sort of manual actuation by the vehicle operator to lift the check valves (of both of the valve assemblies) off of their respective check valve seats, thus allowing oil to flow freely from one side of the hydrostatic closed loop to the other side (i.e., from the “A” port to the “B” port, or vice versa). In the condition described, the motor which normally drives a vehicle drive wheel is then able to turn freely as the vehicle is being towed, without causing the fluid to overheat.
It is, of course, desirable to combine all of the various functions described above into one integrated valve assembly in order to reduce the size and complexity and expense of the pump end cover, or whatever portion of the pump housing contains the valve assemblies. An example of a prior art valve assembly including all of the functions described above, is illustrated and described in U.S. Pat. No. 4,617,797, incorporated herein by reference. In the “multi-function valve” of the above-cited patent, the POR valve portion of the multi-function valve serves as a “pilot” for the high pressure relief valve (“HPRV”) portion. Therefore, although each of the individual functions of the multi-function valve of the above-cited patent has been generally satisfactory in operation, it has been observed that one major disadvantage of this prior art device is that, because the POR valve serves as the pilot for the HPRV, the fluid pressure at which the HPRV opens and begins to relieve pressure is always a constant differential pressure (for example, 500 psi) higher than the POR setting.
Any adjustment (such as by the vehicle operator) of the POR setting will change the HPRV setting accordingly, and excessive adjustment of the POR setting can result in loss of the pilot flow which is required to actuate the HPRV. Finally, in the multi-function valve of the above-cited patent, the POR function requires an increasing pump output pressure in order to generate an increasing rate of flow to destroke the pump, which those skilled in the art will understand to be counter-productive.